Process for producing fine polyethylene powder

ABSTRACT

It has been found that a fine powder of polyethylene, the average particle size of which is not more than 100 microns in diameter, is obtained by polymerizing ethylene at a temperature lower than the melting point of the produced polyethylene, by means of irradiation of an ionizing radiation or free radical initiator, dispersing the thus produced powder polyethylene in a water-miscible liquid dispersing medium which wets the polyethylene but does not substantially dissolve it, and the surface tension of which is not more than 32.5 dyne/cm, to dissociate the agglomerated polyethylene powder particles washing the dissociated polyethylene powder particles with water, and drying the particles.

United States Patent Takehisa et a1.

[451 July 11, 1972 [54] PROCESS FOR PRODUCING FINE POLYETHYLENE POWDER[72] Inventors: Masaaki Takehisa; Shiro Senrui; I-Iayato Nakajima, allof Gunma-ken, Japan [73] Assignee: Japan Atomic Energy ResearchInstitute,

Tokyo, Japan [22] Filed: Feb. 28, 1969 [21] Appl.No.: 803,990

[30] Foreign Application Priority Data March 4, 1968 Japan ..43/13630[52] US. Cl ..260/94.9 F 51 ..cosr 47 02 [58] Field of Search ..260/94.9F

[56] References Cited UNITED STATES PATENTS 3,553,187 l/l97l Takehisa etal ..260/94.9

Primary Examiner-.Ioseph L. Schofer Assistant Examiner-Stanford M. LevinA!rorneyl(urt Kelman [57] ABSTRACT It has been found that a fine powderof polyethylene, the average particle size of which is not more than 100microns in diameter, is obtained by polymerizing ethylene at atemperature lower than the melting point of the produced polyethylene,by means of irradiation of an ionizing radiation or free radicalinitiator, dispersing the thus produced powder polyethylene in awater-miscible liquid dispersing medium which wets the polyethylene butdoes not substantially dissolve it, and the surface tension of which isnot more than 32.5 dyne/cm, to dissociate the agglomerated polyethylenepowder particles washing the dissociated polyethylene powder particleswith water, and drying the particles.

3 Claims, No Drawings PROCESS FOR PRODUCING FINE POLYETHYLENE POWDERBACKGROUND OF THE INVENTION This invention relates to a process forproducing a fine powder of polyethylene the average particle size ofwhich is not more than 100 microns in diameter, which we call finepowderpolyethylene hereinafter.

It was known prior to this invention that polyethylene is produced inpowder form when ethylene is polymerized under pressurization in thegaseous phase, or in a liquid or gas-liquid mixed phase, in the presenceof a solvent in which the produced polyethylene does not dissolve, at atemperature lower than the melting point of the produced polyethylene,by means of irradiation of an ionizing radiation or a free radicalinitiator. And it was known, too, that the thus produced powderypolyethylene has properties superior to those of the polyethylenepowders which are produced from polyethylene of the prior art bysecondary mechanical or physicochemical pulverizing treatment; and wecall this powdery polyethylene powder polyethylene" in order todistinguish it from the above-mentioned conventional polyethylenepowder.

However, this powder polyethylene still has some defects. The powderpolyethylene (produced in the gaseous phase, liquid phase or gas-liquidmixed phase, and separated from the monomeric ethylene or the liquidmedium) easily agglomerates, and the original fine powder state cannotbe easily regained by any simple method. It is thought that suchagglomeration is caused by the co-existing oligomer of ethylene.

Therefore, the particles of the so-called powder polyethylene" are notparticles as polymerized from the gaseous phase or liquid or gas-liquidmixed phase in which the polyethylene does not dissolve, but more orless agglomerated particles, and thus the average particle size of thehitherto known powder polyethylene is of the order of 200 300 microns indiameter, and a fine powder of polyethylene not more than 100 microns indiameter has never been produced.

CROSS REFERENCE TO CO-PENDING RELATED I APPLICATION The invention ofthis application is closely related to the invention of US. Pat. No.3,553,l87.

SUMMARY OF THE INVENTION The purpose of this invention is to provide aprocess for producing fine powder of polyethylene the average particlesize of which is not more than 100 microns. And the purpose is achievedby polymerizing ethylene in the presence of a solvent in which theproduced polyethylene does not dissolve, at a temperature lower than themelting point of the produced polyethylene, by means of irradiation ofan ionizing radiation or a free radical initiator, dispersing the thusproduced powder polyethylene in a liquid organiccompound or an aqueoussolution thereof which has a surface tension not more than 32.5 dyne/cm,and wets the surface of polyethylene, dissolves in water in an amount ofSg/ 100 ml or more, and does not substantially dissolve polyethylene,thereafter separating said dispersion medium, and washing the separatedpowder with water.

The above-mentioned organic compound includes an a alkanol, a ketone andan ether having I 4 carbon atoms, a fatty acid having 1 4 carbon atoms,etc.

The liquid which wets the produced powder polyethylene and subsequentlydisperses it must have a surface tension of notmore than 32.5 dyne/cm.This was established by our coworkers including one of us and isdescribed in the specification of the above-mentioned US. Pat.application Ser. No. 687619.

The liquid organic compound applied to the process of this inventionmust have solubility of at least 5g/ 100 ml in water. If the solubilityin water of the liquid is less than this, it is difficult to remove theliquid from the polyethylene powder only by washing several times withwater. Therefore, it is practifrom the polyethylene powder istheoretically possible.

Some examples of the liquid applicable to the process of this inventionare shown in Table l.

TABLE I Inherent Solubility Liquid surface in water Surface tension oftension (g/ ml) water solution (dyne/cm) (dyne/cm-wt% cone.) Methanol22.6 00 32.5-54 Ethanol 22.6 m 32.5-3 2 Isopropanol 2 l .4 325-14n-Propanol 23.7 m 32.5-l 1.5 n-Butanol 24.6 8 32.5-5 sec-Butanol 23.512.5 32.5-6 tert-Butanol 19.3 at 32.5-6 Acetone 23.7 I 32.5- 49 Ethylether I 7.0 7.5 Acetic acid 27.7 x 3 2.5-76 Propionic 26.0 32.5-5 l acidAll these organic liquids can be used for the process of this inventioneconomically on an industrial scale.

Some of these liquids can be used in the step of polymerization ofethylene as the solvent to form a liquid phase or a gasliquid mixedphase together with ethylene. If these liquids are used in this way, thepurpose of this invention is achieved by simply washing the slurry ofpowder polyethylene resulting from the polymerization step with water.

The fine-powder polyethylene produced in accordance with the process ofthis invention has an average particle size of not more than 100microns, and does not agglomerate. Because of its large specific surfacearea, the fine-powder polyethylene can be used as a very activeadsorbent for organic vapor and liquids. It can be used as a blendingmaterial for other plastic materials such as polyester resin in order toimprove their physical properties. The fine-powder polyethylene issuitable for coating metal substrates by depositing it thereon from adispersion thereof in a liquid or from the free powder state.

DETAILED DESCRIPTION OF THE INVENTION Now the invention is illustratedby way of working examples, but it will be understood that the inventionis by no means restricted to these specific modes of working.

Example I A partly agglomerated powder polyethylene, the mean molecularweight of which is 8.4 X 10*, and the density of which is 0.933 g/ml,was produced by circulating ethylene through a reactor having a capacityof 10 liters at a flow rate of 9 kg/hr at 30 C, the pressure inside thereactor being maintained at 400 kg/cm all the time; and irradiating theethylene in the reactor with gamma radiation from cobalt-60 of 10 Curiesat a dose rate of 3.7 X 10 rad/hr. This powder polyethylene which ishereinafter called polyethylene A was made finer by the process of thisinvention.

Twenty-five grams of polyethylene A was dispersed in 1 liter of methanolby means of a small mixer, and thereafter was separated from themethanol by filtration. The thus treated radiation-polymerizedpolyethylene A was then dispersed in water and filtered so as to removemethanol, and was dried in air kept at 80 C. The driedradiation-polymerized polyethylene A was, without any pulverizingtreatment applied, subjected to screening analysis by using the ASTMstandard sieve and a Ro-tap type sieve shaker for 20 minutes inaccordance with the method of JIS (Japanese Industrial Standards) 6002.The result is shown in Table 2.

TABLE 2 No. of sieve (mesh) Opening of the sieve Wt.% of fine (microns)powder 45-120 350- 4.24 l20-l70 125-88 7.63 [70-230 88-62 37.30 230-32562-44 41.10

325 up not more than 44 Average particle size: 70.0 microns There was nofraction remaining on the sieve No. 45.

Example 2 In accordance with the operation as described in Example 1,radiation-polymerized polyethylene A was made finer by using ethyl etherinstead of methanol. The water-solubility of ethyl ether is 7.5 g/ 100ml at 20 C. The result is shown in Table 3.

TABLE 3 No. of sieve (mesh) Opening of the sieve Wt.% of fine (microns)powder 45-120 350-125 3.79 120-170 125-88 9.51 170-230 88-62 67.00230-325 62-44 15.79 passing 325 not more than 44 3.92

Average particle size: 79.3 microns TABLE 4 No. of sieve (mesh) Openingof the sieve Wt. of fine (microns) powder 45-120 350-125 9.16 120-170125-88 17.00 170-230 88-62 25.70 230-325 66-44 22.77 Passing 325 notmore than 44 25.40

Average particle size: 76.2 microns There was no fraction remaining onthe sieve No. 45.

Example 4 Ethylene was circulated through a reactor having a capacity of10 liters at a flow rate of 27 kg/hr at 30 C while the pressure insidethe reactor was maintained at 400 kglcm and at the same time a 40 weightpercent acqueous solution of tertbutanol was circulated through saidreactor at a flow rate of 60 kg/hr. The content of the reactor wassubjected to the irradiation of gamma radiation from a cobalt-60 sourceof 10 Curies at the dose rate of 2.0 X rad/hr. Thus powder polyethylenewas produced and taken out of the reactor in the form of a slurrycomprising suspended powder polyethylene and tert-butanol-water medium.The molecular weight of the polyethylene was 2 X 10, and its density was0.940 g/cm. This powder polyethylene is hereinafter called polyethyleneB.

A small amount of the polyethylene B obtained in the form of slurry in50 weight percent aqueous solution of tert-butanol was filtered, washedwith water and dried in the same way as in Example 1. The thus obtainedfine-powder polyethylene was subjected to the sieving analysis as inExample 1. The result is shown in Table 5. Incidentally the polyethyleneB directly separated and dried from the slurry coagulates and givespartly agglomerated so-called powder polyethylene.

Average particle size: 55.3 microns There was no fraction remaining onthe sieve No. 45 Example 5 Ethylene was charged into a reactor having acapacity of 500 ml up to a pressure of 400 kg/cm at 40 C, and waspolymerized by injecting azo-bis-isobutyronitrile as the free radicalinitiator. Agglomerated powder polyethylene having a molecular weight of3 X 10 and a density of 0.931 g/cm was obtained. This is hereinaftercalled polyethylene C.

Polyethylene C was made finer in the same way as in Example 1. That isto say, 25 g of polyethylene C was dispersed in 1 liter of methanol bymeans of a small mixer, and separated by filtrationand washed with waterand dried as in Example 1. The result is shown in Table 6.

Average particle size: 68.6 microns There was no fraction remaining onthe sieve No. 45

Example 6 Through a tubular reactor comprising a stainless steel tube 10mm in inside diameter, 20 mm in outside diameter, 90 m in length, and 7liters in volume coiled into a spiral 1 m in diameter, amethanol-ethylene mixture (43.5 weight percent ethylene) was passed at aflow speed of 4.5 m/min at the pressure of 400 kg/cm and the passingethylene was irradiated with gamma radiation from a cobalt-60 source of10 Curies placed in the center of the coil at the dose rate of 2.5 X 10rad/hr at 20 C. A powder polyethylene, the molecular weight of which is2.1 X 10", and the density of which is 0.955 g/cm, was obtained in thestate of slurry. This powder polyethylene is hereinafter calledpolyethylene D.

The polyethylene D obtained in the form of slurry in methanol wasseparated by filtration, and a small amount thereof was washed withwater and dried as in Example 1. The result of the sieving analysis ofthis product as carried out as in Example 1 is shown in Table 7.

Average particle size: 51.5 microns There was no fraction remaining onthe sieve No. 45.

By way of comparison, some examples are shown in which the so-calledpowder polyethylene was treated in the same way as in the above examplesbut using liquids other than the liquid as defined above.

Example 7 Twenty-five grams of the same polyethylene A as used inExample 1 was dispersed in 1 liter of n-hexane by means of a smallmixer, and was separated from the liquid by filtration. The thus treatedpowder polyethylene was then dispersed in water by the same mixer so asto remove hexane, and was filtered and dried in an air bath kept at C.The dried powder polyethylene was in the agglomerated state and was notfine powder. The surface tension of n-hexane is 18.4 dyne/cm at 20 C,and its water-solubility is 0.014 g/l00 ml 15 C); that is, the liquid isscarcely soluble in water, and therefore, it is not easily removed fromthe polyethylene powder by simple washing treatment.

Example 8 The operation of Example 7 was repeated by using isobutylacetate instead of n-hexane. The resulting powder polyethylene was inthe agglomerated state and was not fine powder.

The surface tension of isobutyl acetate is 23.7 dyne/cm C), and itssolubility in water is 0.63 g/ 100 cc C). This liquid is not easilyremoved by washing with water, either.

Example 9 The operation of Example 7 was repeated by using 1 liter of vdiameter, wherein ethylene is polymerized at a temperature lower thanthe melting point of the produced polyethylene by means of ionizingradiation or a free radical initiator, the improvement consistingessentially of dispersing the thus produced polyethylene agglomerates ina dispersing medium consisting of an aqueous solution of an organiccompound. said organic compound being a ketone, or an ether, each hav'ing 1 to 4 carbon atoms, until the agglomerates of polyethylene aresubstantially dissociated into line powder having the said averageparticle size, said dispersing medium having a surface tension of nomore than 32.5 dyne/cm, wetting the surface of the polyethylene withoutsubstantially dissolving the same; separating the dispersing medium fromthe polyethylene; washing the separated polyethylene powder with waterto remove residual dispersing medium therefrom; and drying the washedpowder at a temperature lower than the melting point of the polyethylenepowder.

2. The process of claim 1 wherein the organic compound is acetone, orethyl ether.

3. The process of claim 1 wherein the polymerization is carried out in aliquid or gas-liquid mixed phase in the presence of the dispersingmedium as the polymerization medium, and said dispersion takes place asthe polymerization proceeds.

2. The process of claim 1 wherein the organic compound is acetone, orethyl ether.
 3. The process of claim 1 wherein the polymerization iscarried out in a liquid or gas-liquid mixed phase in the presence of thedispersing medium as the polymerization medium, and said dispersiontakes place as the polymerization proceeds.